Extension Ag Update
March/April 2001
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Research Results

Discriminating Weed Sprayer

Dr. Lei Tian, University of Illinois, Ag Eng. Dept., 1304 W. Pennsylvania Ave., Urbana, IL, 217-333-7534, lft@sugar.age.uiuc.edu. Source: Ag Eng. News

Several manufacturers are looking at a prototype sprayer developed at the University of Illinois. The "smart sprayer" uses video cameras mounted ahead of the spray boom to determine the size and density of weeds between the crop rows and creates a weed map and application prescription. The two-dimensional image sensor array has higher resolution than the WeedSeeker's photo sensor.

The University of Illinois smart machine sprays a 10 percent rate of herbicide across the entire field but automatically calculates and increases the rate at each spray nozzle to match weed size and density. "Young weeds may only require a 20 percent rate of the herbicide, whereas areas with large numbers of large weeds may need a full rate," explains Dr. Lei Tian, professor of ag engineering at the University of Illinois. The machine can apply a different variable rate of herbicide from each nozzle, whereas the WeedSeeker nozzles can only be turned on or off.

"We've found that many fields are only 10 to 30 percent infested with weeds that need to be sprayed. Our average herbicide savings has been 52 percent," Tian says. He has been using Roundup Ready herbicide on Roundup Ready crops, but a selective herbicide could be used on conventional crops instead. Like the WeedSeeker, the University of Illinois machine does not distinguish between crops and weeds or different weed species at this time.

Green Manure Crops Enhance Natural Disease Control in Soils

Linda Kinkel Plant Pathologist, University of Minnesota, lindak@puccini.cdl.umn.edu, 612- 625-0277, Source: Jack Sperbeck, sperb001@umn.edu, 612-625-1794

New research results from the 2000 growing season show that green manure crops—buckwheat, oats and sorghum-sudangrass—significantly reduced root disease in alfalfa. The green manure crops enhanced the natural disease-suppressive activity of the soil microorganisms, according to Linda Kinkel, University of Minnesota plant pathologist. In the first two years of the trials, Kinkel and colleagues focused on identifying green manure crops that could enrich the activity of natural soil antagonists.

During the third year—the 2000 growing season—they initiated studies on disease control potential of the best green manures. They found significant control of Phytophthora root rot in alfalfa preceded by a single planting of green manure crops. Buckwheat, oats and sorghum-sudangrass crosses all enhanced the soil's natural disease control activity and provided significant disease control. "Not all green manure crops enhance natural antagonists of soil pathogens," Kinkel says. "Of 13 green manure crops we've studied, some do nothing to enhance soil antagonists." So although crop rotations that include green manures help reduce soil pathogen levels and prevent disease build-up, they don't necessarily enhance natural disease suppression activity. Kinkel says oats, buckwheat, canola and sorghum-sudangrass have been the most successful crops for enhancing activity of natural soil antagonists to control pathogens.

Much of Kinkel's work has involved diseases of potatoes. "Using green manure crops should increase profits, since they involve no added costs. In the longer term, we'd like to develop three- to five-year cropping systems involving green manures and crops such as potatoes, corn, green beans and alfalfa, where the green manures would enhance natural disease control. These integrated cropping systems could provide broad-based disease control for all crops." "Some inoculative biological controls work incredibly well," Kinkel says. "But many fail completely for reasons that are difficult to discern."

Bringing Back Native Soil Fungi

When you think of endangered species, you never think of soil fungi. Yet the fungi that make plants hardier have had their numbers greatly reduced by the intensive agriculture practiced in the United States since the 1950s. ARS scientists are trying to figure out how to put these beneficial soil fungi back, as farmers make the transition to using less chemicals. One approach researchers are evaluating is to mix the fungi—called mycorrhizae—into potting soil planted with grass or other host plants. Farmers would buy these "inoculated" seedlings and plant them in compost. Then, after the fungi multiplied, farmers would apply the colonized compost with manure spreaders.

The mycorrhizal fungi are beneficial organisms that live on plant roots and help them extend their reach for water and fertilizer. In exchange, the plant gives the fungi the sugar they need to grow. The most common type lives inside root cells and extends long, rootlike threads in the soil.

Farmers today have to rely on whichever of these native fungi survived years of chemical use--from synthetic fertilizers to fungicides. An ultimate goal is to produce the fungi in large quantities efficiently and economically, without host plants. They would then be applied as a biofertilizer before planting.

Building Wheats with Multiple Resistance to Leaf Rust

Gina Brown-Guedira, ARS Plant Science and Entomology Unit, Kansas State University, Manhattan, Kan, 785-532-7260, gbg@ksu.edu, Source: ARS News Service, Linda McGraw, 309 681-6530, mcgraw@ars.usda.gov

Genetic markers—tools of modern biotechnology—are being used by scientists to fortify wheat with longer-lasting resistance to leaf rust, a disease that costs wheat farmers millions annually. ARS plant geneticist Gina Brown-Guedira in Kansas, is building gene complexes using markers closely linked to leaf rust resistance. The markers are made of genetic material called DNA.

Brown-Guedira is combining leaf rust resistance found in two ancestors of modern wheat: Aegilops tauschii (also known as goatgrass), a weedy wheat relative found from Afghanistan to Syria, and Triticum timopheevii from Iran, Iraq and Turkey. Ultimately, genes from these ancestors can be combined and moved into germplasm from which new resistant wheat varieties can be developed.

Leaf rust is caused by a fungal pathogen called Puccinia triticinia. In addition, leaf rust seriously affects the milling and baking qualities of wheat flour. In the past, wheat-breeding programs have released resistant varieties with only a single leaf rust resistance gene. A few years later, these varieties usually begin to lose their effectiveness against the rapidly changing fungus. The result is a boom and bust cycle of wheat disease for farmers in the major wheat growing areas of the world. Scientists currently must use time-consuming classical genetic studies to determine if a plant has more than one resistance gene. In contrast, Brown-Guedira can look for the DNA markers at any stage of plant growth without having to infect plants with the fungus. Because the markers are closely linked to the resistance genes, there is a good chance the genes are present.